Energy efficiency has been a key design constraint for microprocessor development teams since the late 1990s. The fundamental technological issues that have led to this point are quite well understood at this time by industry and academia. Although active (or dynamic) and passive (or standby) components of the net power equation are of concern, in recent years the latter (leakage) aspect of chip power has been escalating at a much faster rate than active power. IEEE Micro, in putting together a special issue on this important theme, decided to focus on two recently held (and relatively new) conferences: Cool Chips VIII, held April 2005 in Yokohama, Japan, and the fourth Annual Austin Conference on Energy Efficient Design (ACEED), held in March 2005 in Austin, Texas. The papers in the issue, from those conferences, focus on energy efficient design, and the Cell processor.